Electronic spectra oxygen hydrates

When solutions of CdS colloids containing no additional electron and hole acceptor in the solution, are exposed to a high intensity laser flash, a rather large absorption of an intermediate is observed around 700 nm, similarto that described for the laser excitation of Ti02 in the previous section. The absorption spectrum of the intermediate is given in Fig. 9.17 [52]. It is not due to trapped electrons and holes but it is identical with to the well-known spectrum of hydrated electrons as proved by radiolysis experiments [52]. The half-life of the hydrated electrons is a few microseconds. In the presence of typical hydrated electron scavengers, such as oxygen, acetone or cadmium ions, the decay of the intermediate became much faster. 

In very rare cases in which photoionization is the only photoprocess, the absorption observed in a nitrous oxide-saturated solution will be that of the cation radical. However, generally, other processes such as intersystem crossing also occur in parallel. In the presence of oxygen, the hydrated electron and the triplet will be scavenged at diffusion-controlled rates, and the absorption observed in the oxygenated solution will be due to the cation radical. Under these conditions, the cation radical spectrum is easily determined. The molar absorption coefficient of the cation radical can also be calculated using the hydrated electron as an internal standard. The molar absorption coefficient for sulphacetamide cation radical was determined in this manner (Land et al 1982) and later confirmed by pulse radiolysis (see Section 12.2.2.6). 

Fig. 5.11 comprises most of the important oxygen species and reactions in water. The small box with the continuous line comprises the chemical regime reduced in oxygen-free water. Because in natural waters oxygen is always dissolved, ROS such as peroxides are produced and destroyed in waters under the influence of light (larger dotted box in Fig. 5.11). But besides hydrogen (H" ) and hydroxide ion (OH ) another fundamental species of aqueous solutions exists. This is the hydrated electron aq, also written as H20 , and first postulated by radiation chemists Gabriel Steirif in 1952 and characterized in 1962 by recording its absorption spectrum (Stein 1968, Hart and Anbar 1970, Hughes and Lobb 1976). The solvated...

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